As is known, a substrate such as a printed circuit board on which electronic components are placed requires that the components be soldered to the board. A viscous material, such as a non-conductive or conductive adhesive, solder paste or another silicon type viscous material is often deposited on the substrate before the component is placed on the substrate. Screen printing machines that are commercially available from a number of sources are used to automatically deposit the viscous materials through a mesh or metallic screen or stencil onto the substrate.
Generally, the solder pastes used in screen printing in the electronics industry are heterogeneous materials, the components of which have different densities, and are composed of metallic materials and organic or flux materials. The mass of the metallic portion of the solder paste represents approximately 85% to 90% of the total mass, with a density of 8 to 12 according to the metals used. It is understood that the term density means the weight as compared to 1 liter of water. In volume, the metallic portion represents only approximately 50% of the total volume. The organic material, also called flux, has a density of approximately 1.
The solder pastes described above are made up of metallic microspheres joined by the flux or organic material. This viscous flux comprises rheologic agents, adhesive agents and cleaning agents which affect the process of assembling components on printed circuit boards, The process, which is well known, involves:
deposition by screen printing of solder paste contacts onto selected portions of the substrate; PA1 placing of component leads on the paste contacts, the adhesive agent of which holds the components to the board; and PA1 reflowing the solder paste in a furnace or oven, which causes the coalescence of the metallic microspheres, and, when cooled, results in the component being fixed on the board at the proper location.
The function of the alloy included in the solder paste is to provide the supply of metal necessary to ensure electrical interconnection between the leads of the components and the printed circuit by soldering. The organic materials in the paste must disappear at the conclusion of the soldering operation. Nevertheless, there is normally a residue of the organic materials which must be cleaned with water or with solvent, which is both costly and polluting.
Solder pastes reportedly resulting in a low residue have been developed. In these pastes, the organic part has substantially the same value in volume terms as in the previous pastes described above. Light solvents with low boiling points can be introduced to provide proper rheology or flow characteristics. Because of the low boiling points, these solvents become volatile more rapidly during a pre-heating step, which generally precedes the reflow step described above during the assembly of components onto printed circuit boards. At the end of the reflow step there thus remains little residue. In order to provide a satisfactory adhesive capacity, the light solvents described above are combined with adhesive resins, which become volatile or sublimate in the reflow step.
In addition, in these low residue solder pastes, the cleaning agents used for preparation of a surface which is suitable for producing satisfactory inter-metal connections occupy a very small part of the total volume of the paste. As the overall efficiency of the cleaning must not change, the volume efficiency of the active cleaning constituent has to increase in equal proportion to the decrease in residue.
These developments in low residue solder pastes result, on the one hand, in a greater dilution of the active cleaning constituents in the paste and, on the other hand, in a greater volatility of the additional solvents used. It is therefore necessary that an extremely homogeneous distribution of the active cleaning constituent be obtained within the volume of each deposit of solder paste when the solder paste is applied to the substrate. If this is not achieved, deposits of the solder paste will be obtained in which the efficiency of cleaning, for example, will not be identical for adjacent areas. Certain areas will have too much cleaning constituent applied, resulting in cleaning and residue problems. Other areas will not have sufficient cleaning constituents, and therefore the soldering obtained will be of poor quality.
The high degree of solvency and solubility of the additional solvents used in the low residue solder pastes results in the evaporation thereof while printed circuit boards are produced. In prior art modes of deposition, the material is dragged by means of an inclined wiper (see the prior art system shown in FIG. 1 and described below) in the open air and therefore the evaporation problem is not solved. The evaporation results in a change in the rheology of the solder paste during production. In extreme cases, the solder paste may become too dry and no longer pass properly through the apertures in the stencil.
These problems are exacerbated when production requirements necessitate high-speed screen printing, for example at 200 millimetres per second as opposed to 20 to 50 millimetres per second. To counteract these problems, thixotropic additives are introduced and combined with the other solvents. Evaporation of the base solvents therefore modifies the possible speed of deposition. By way of example, an evaporation of 1% from the volume of solder paste completely changes the rheology and makes screen printing very difficult, if not impossible.
Another of the problems caused by the known technology is the control of the wear and tear on the wiper system. Progressive erosion of the active edge of the wiper by rubbing alters the intrinsic qualities of the paste applied, and that of the depositions, because of the uncontrolled and random retrieval of a certain quantity of microspheres of the metallic portion of the solder with each wipe. In fact, the wipers are normally only changed when the poor quality of the deposition is a noticeable consequence of their wear.
In the prior art, two types of wipers have been commonly used. The first type of wiper is a rubber or polyurethane type. The hardness of this type of wiper varies generally between 70 to 90 Shore. This wiper has the advantage of good deformation by virtue of its low degree of hardness and its flexibility, and therefore good sealing is produced. It has the inconvenience of deforming during passage over the apertures in the stencil. For apertures where the dimension parallel to the wiper is less than 0.5 mm, this is not a major problem. However, where the apertures have dimensions parallel to the wiper greater than this value, the deposit is hollowed out. Where deposits are larger than 3 mm, they are completely dragged off again.
The second type of wiper is a metallic type. The advantage of this type of wiper is its ability to maintain rigidity and therefore not allow the deposit to be hollowed out. The hardness of this type of wiper, however, despite its flexibility, does not allow for perfect sealing with the stencil. The hardness of the metallic wiper sometimes exceeds that of the stencil and therefore often scratches the stencil resulting in encrustation of solder microspheres. The excessive pressure of the metallic wiper can also cause crushing of the tinlead spheres, this alloy being much softer than the steel wiper.
FIG. 1 shows a prior art implementation for depositing a viscous material onto a substrate 1 through a stencil or a screen 2 provided with apertures or openings 3, by means of a wiper 4. The material to be deposited is labeled 5.
In FIG. 1, standard wiper 4, inclined at an angle which can vary from 60.degree. to 45.degree. with respect to the substrate 1, fulfils at least two functions at the same time. First, it drags the material to be deposited over the stencil (in FIG. 1, with a force in the direction of the arrow 4A). Second, it transfers the material through the apertures or openings 3 in the stencil or screen 2 (in FIG. 1, with a force in the direction of the arrow 4B).
The force of transfer, however, can only be exerted if there is displacement of the wiper 4. Furthermore, this force is not constant over the whole length of the wiper 4, but rather is at its maximum at the ends of the wiper 4 and decreases along the length thereof Because of this differential in force, the result of the transfer is directly linked to the viscosity of the material (which changes quickly), the force of transfer resulting from the sloping of the wiper 4, and from the movement of the wiper 4.
At the point of contact between the wiper 4 and the stencil 2, the wiper 4 fulfils three functions: (1) sealing between the stencil and the wiper; (2) wiping the stencil 2, which allows removal of the surplus material; and (3) contact between the stencil 2 and the substrate 1, there being no contact downstream and upstream of the wiper 4.
The fact that a single wiper 4 fulfills all of these functions makes independent action with respect to each of these functions impossible within the prior art technology. Moreover, the prior art technology has several disadvantages. Referring to FIG. 1, the material to be distributed through the apertures 3 is always downstream of the wiper 4. As a result, as shown in FIG. 1, when the filling of the aperture 3 takes place, it is always in a zone where the stencil 2 is not in contact with the substrate 1. Therefore, the material can be pushed in between the stencil 2 and the substrate 1 (designated as item 5A in FIG. 1), making on the one hand undesirable lines on the substrate 1, and on the other hand fouling the stencil 2, which must be cleaned frequently. In addition, because the material is dragged by means of the wiper 4 in the open air, evaporation of the constituent parts of the material may occur.
Moreover, the transfer efficiency of a system can be simply characterized by the following coefficient, K: ##EQU1## wherein T is the contact time, P is the pressure applied to the material being transferred, and V is the viscosity of the material being transferred. With prior art squeegee printing described above, transfer efficiency cannot be controlled because all of the factors, T, P and V are variable.
WO 96/20088 filed by the Ford Motor Company relates to a method and an apparatus for distributing a viscous material by compression thereof through the apertures of a stencil. The apparatus comprises a reservoir receiving a charge of viscous material; a pressure is exerted on the viscous material in the reservoir. The reservoir is linked via a conduit to a distribution nozzle or compression head having a conical internal shape with baffle plates. The distribution nozzle is provided with a rectangular distribution slit delimited by two wipers disposed in opposite directions, slightly inclined with respect to the vertical. The two wipers bear against the stencil and keep it in contact with the substrate in the zone between them. The aim of this apparatus is to allow implementation of high-speed screen printing.
It appears that the technology disclosed by WO 96/20088 not only does not permit the resolution of the problems described previously, but moreover accentuates them. Indeed, the viscous material has to be placed in a reservoir which is an integral part of the system. The viscous material must, following the reservoir, be pushed under pressure towards the nozzle. The system according to WO 96/20088 has disadvantages, in particular with regard to the cleaning of the conduit from the reservoir to the nozzle. Further, the conical internal shape of the nozzle and the baffle plates with which it is provided, in theory provided to guide and equalize the pressure, will have the effect of laminating the paste. Such an effect is hardly compatible with the heterogeneous nature of the paste and the difference in density of the metallic parts and the flux. Furthermore, this laminating creates a significant risk of separating the components of the paste and thus results in deposits of unequal quality. In addition, according to WO 96/20088, the nozzle and wipers bear upon the stencil either under the effect of a pressure independent of the pressure applied to the material in the nozzle or under the effect of springs acting on the wipers.